Microbial Inoculant Reduced Cadmium Accumulation in Triticum aestivum L. and Modulated Soil Bacterial Community Composition and Soil Enzyme Activities

Cadmium (Cd) contamination accelerates soil degradation and threatens the safe utilization of agricultural land, making sustainable remediation strategies urgently needed. To evaluate the remediation efficacy of a microbial inoculant (MI) composed of Bacillus subtilis, Bacillus thuringiensis, and Bacillus amyloliquefaciens in Cd-contaminated wheat soils, we conducted a field experiment. The results showed that, compared with the control (CK), MI treatments increased soil pH and soil organic matter (SOM) and enhanced the activities of urease, sucrase, dehydrogenase, and alkaline phosphatase by 3.83–249.28%, 0.54–1.08%, 5.61–109.68%, and 2.76–13.38%, respectively. With MI application, Cd shifted from the exchangeable and Fe-Mn oxide-bound fractions to the residual fraction, a key pathway reducing Cd bioavailability. At the wheat maturity stage, MI treatments reduced Cd content in wheat grains by 12.83–33.15% compared with CK. High-throughput sequencing revealed increased relative abundances of bacteria tolerant to heavy metals and bacteria associated with heavy metal remediation, including the phyla Pseudomonadota, Acidobacteriota, Bacillota, and Actinomycetota and the genera Pseudomonas, Arthrobacter, and Bacillus. Mantel tests indicated that soil pH, SOM, enzyme activities, and Cd accumulation were most strongly correlated with bacterial community composition. Overall, MI demonstrates practical feasibility for remediating Cd-contaminated wheat-field soils.